Method for manufacturing thin film transistor

ABSTRACT

A method for manufacturing a thin film transistor is disclosed. To solve the problems of leakage current generated at the grain boundary which passes through a source and a drain and the difficulty in obtaining stable device characteristic owing to the change of the lengths of the channel region and the offset region according to the degree of the overlay misalignment, the method includes the steps of forming a polysilicon layer on a substrate, forming an impurity-containing layer on the polysilicon layer, forming a prescribed pattern by patterning the impurity-containing layer, flowing the impurity-containing layer by heat treating, and crystallizing the polysilicon layer by annealing using a prescribed light source, wherein the flowed impurity-containing layer functions a lens during crystallizing the polysilicon layer to generate a partial temperature difference between the portion of the polysilicon layer where the impurity-containing layer is formed and the portion of the polysilicon layer where the impurity-containing layer is not formed.

BACKGROUND OF THE INVENTION

The present invention relates to a method for manufacturing a thin film transistor, and more particularly to a method for manufacturing a thin film transistor which is applicable to a SRAM (static random access memory) and liquid crystal display devices.

In the conventional method for manufacturing a thin film transistor, a bottom gate and a top gate are used. Polysilicon for forming a body layer is manufactured by enlarging the particle size using solid phase growing method. The solid phase growing method for the body polysilicone layer according to the conventional method is carried out by heat treating at about 600° C. for about 24 hours.

Recently, an attempt to increase the crystallization degree by RTA (rapid thermal annealing) is made. However, this increases production cost and is difficult to proceed in batch type.

A plasma H₂ passivation used to be applied to obtain on/off current ratio (I_(on) /I_(off)) of 10⁶ or above.

The conventional method for manufacturing a thin film transistor will be described below referring to attached FIGS. 1a-1c.

First, a gate electrode 2 is formed by depositing a conductive material such as polysilicon on an insulating layer 1 formed on a substrate (not shown) and patterning; through photolithography as shown in FIG. 1a.

After then, a gate oxide 3 and a body polysilicon layer 4 are sequentially formed by CVD (chemical vapor deposition) on the whole surface of the insulating layer 1 in which the gate electrode 2 is formed, and channel ions are implanted.

As shown in FIG. 1B, photoresist 5 is coated on the body polysilicon layer 4 and is selectively exposed and developed to form a prescribed photoresist pattern to define a channel region and an offset region on the body polysilicon layer 4. At this time, the region covered with the photoresist 5 becomes the channel region and the offset region. Ion implantation is carried out to form an LDD (lightly doped drain) region 11 using the photoresist pattern 5 defining the channel region and the offset region as an ion implantation mask.

As shown in FIG. 1C, the photoresist pattern 5 defining the channel region and the offset region is removed and the photoresist 6 is coated again, selectively exposed and developed to form a photoresist pattern 6 defining a source region and a drain region. Ion implantation is carried out using this photoresist pattern 6 as a mask to form the source region and the drain region 12 to complete the manufacture of the thin film transistor as shown in FIG. 1C.

FIG. 2 is a schematic cross-sectional view of the channel region formed on the body polysilicon layer 4 of the thin film transistor manufactured by the conventional method.

According to the conventional method, a problem of leakage current generation by the grain boundary A which passes through the source S and the drain D as shown in FIG. 2 occurs.

Moreover, since the offset region and the source and the drain region are defined using the photoresist, the length of the channel region and the length of the offset region are changed according to the degree of overlay misalignment, and stable device characteristic could be hardly obtained.

SUMMARY OF THE INVENTION

An object of the present invention is to solve the above-mentioned and other problems encountered in the conventional methods and to provide a method for manufacturing a thin film transistor having high reliance and high reproductivity.

To accomplish the above and other objects, there is provided in the present invention a method for manufacturing a thin film transistor comprising the steps of forming a polysilicon layer on a substrate, forming an impurity-containing layer on the polysilicon layer, forming a prescribed pattern by patterning the impurity-containing layer, flowing the impurity-containing layer by heat treating and crystallizing the polysilicon layer by annealing using a prescribed light source.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings in which:

FIGS. 1a through 1d are sectional views for explaining the conventional method for manufacturing a thin film transistor.

FIG. 2 is a schematic cross-sectional view at the portion of the channel of the thin film transistor manufactured by the conventional method.

FIGS. 3a through 3f are perspective views for explaining a method for manufacturing a thin film according to the embodiments of the present invention.

FIG. 4 is a schematic cross-sectional view at the portion of the channel of the thin film transistor manufactured by the method of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in detail below referring to the attached drawings.

FIGS. 3a-3f are views for illustrating the method for manufacturing the thin film according to the present invention.

First, a gate electrode 20 is formed by depositing a conductive material such as polysilicon on an insulating layer 10 formed on a substrate (not shown) and patterning it using photolithography as shown in FIG. 3a.

After then, a gate oxide 30 (gate insulating layer) and a body polysilicon layer 40 are sequentially formed by CVD (chemical vapor deposition) method on the whole surface of the insulating layer 10 where the gate electrode 20 is formed, and ion implantation 70 is carried out for forming a channel region.

As shown in FIG. 3b, an impurity-containing layer such as BPSG 80 (borophospho-silicate glass in case of a P-type thin film transistor) or PSG 80 (phospho-silicate glass in case of an N-type thin film transistor) is deposited on the body polysilicon layer 40 using the CVD method. The BPSG layer (or the PSG layer) 80 is formed while controlling the thickness thereof according to the wavelength of the light source to be used in the following crystallization process. It is preferred that the BPSG layer 80 is formed by washing using SC-1 and HF after the channel ion implantation.

As shown in FIG. 3c, photoresist is coated on the BPSG layer 80 (or the PSG layer) and is patterned as photoresist pattern 90 photolithography to define a source region and a drain region. At this time, the portion covered with the photoresist pattern 90 becomes the source region and the drain region, while the body polysilicon layer portion exposed between the source region and the drain region becomes the channel region.

Next, as shown in FIG. 3d, the BPSG layer (or the PSG layer) 80 is etched using the photoresist pattern 90 as a mask. After removing the photoresist pattern 90, heat treatment is carried out for flowing the BPSG layer 80 (or the PSG layer) as shown in FIG. 3e. The body polysilicon layer 40 in the channel region is crystallized by annealing using a prescribed light source. At this time, RTP (rapid thermal process) can be used or a focused laser can be used as the light source.

Meanwhile, since flowability of the BPSG (or the PSG) is changed according to the concentration of boron (or phosphorous) in the BPSG (or PSG), and the optimized radius of curvature of the flowed state of the BPSG (or PSG) is changed according to the wavelength of the light source used when crystallizing the channel region, the time and temperature of the heat treating of the BPSG layer should be appropriately controlled to obtain desirable flowability and the desired radius of curvature.

The impurity in the BPSG layer (or the PSG layer) 80 diffuses into the body polysilicon layer 40 during the heat treatment to form the source and the drain regions while flowing the BPSG layer 80 (or the PSG layer) to form the shape as shown in FIG. 3e. Therefore, during crystallization of the body polysilicone layer 40, i.e. during forming the channel region, the BPSG layer (or the PSG layer) 80 plays the role of a micro-lens to give the temperature distribution as illustrated in FIG. 3f.

The partial temperature difference affects the nucleation and the growing rate during the crystallization of the amorphous silicon, and this results in the particle size at the interface of the source and the drain region and the channel region being larger than that at the channel region.

In conclusion, the grain boundary which passes through the boundary of the source and the drain region with the channel region can be effectively reduced or increased under the given thermal budget.

FIG. 4 is a schematic cross-sectional view at the portion of the channel of the thin film transistor manufactured by the method of the present invention. No grain boundary which passes through the source region S and the drain region D is observed due to the partial temperature difference during the crystallization as described above.

During the process illustrated in FIG. 3d, ion implantation 100 to form an LDD structure could be carried out before heat treating the BPSG layer 80. In this case, if the ion implantation 100 is followed by the BPSG flowing, the offset region is defined by the difference in the lateral dimension range of the BPSG layer before and after the flowing, as shown in FIG. 3d and FIG. 3e.

The ion implantation for forming the source and the drain region, which has been carried out in the conventional method, can be replaced with the auto doping of boron (or phosphorous) from the BPSG layer (or the PSG layer) during the flowing of the BPSG layer (or the PSG layer) or during the annealing for crystallizing the channel region. In this method, the mask process for patterning the BPSG layer (or the PSG layer) is added, while the ion implantation process is eliminated. This is advantageous in simplifying the manufacturing process.

As described above, according to the present invention, the leakage current owing to the grain boundary which passes through the source region and the drain region can be largely reduced and the off current can be lowered. And since the source and drain region can be self-aligned, the change in channel length due to the overlay misaligned which may occur when forming the source and the drain region by photolithography using photoresist, can be basically eliminated.

Moreover, in case of applying the LDD structure, the length of the offset region due to the difference in the lateral dimension region of the BPSG layer (or the PSG layer) before and after the flowing can be reproductively controlled.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. 

What is claimed is:
 1. A method for manufacturing a thin film transistor comprising the steps of:forming a polysilicon layer on a substrate; forming an impurity-containing layer on said polysilicon layer; forming a prescribed pattern by patterning said impurity-containing layer; flowing said impurity-containing layer by heat treatment, thereby forming a lens on said polysilicon layer; and crystallizing said polysilicon layer by annealing, wherein the crystal size of a first portion of said polysilicon layer where the lens is not formed is different from that of a second portion of said polysilicon layer where the lens is formed.
 2. A method for manufacturing a thin film transistor as claimed in claim 1, wherein the lens causes a partial temperature difference between the first portion and the second portion of said polysilicon layer during crystallizing said polysilicon layer so that the crystal size of the first portion is smaller than that of the second portion.
 3. A method for manufacturing a thin film transistor as claimed in claim 1, wherein said annealing is performed by using a prescribed light source.
 4. A method for manufacturing a thin film transistor as claimed in claim 1, wherein said impurity-containing layer is either a borophospho-silicate glass or phospho-silicate glass.
 5. A method for manufacturing a thin film transistor comprising the steps of:forming a gate electrode on a substrate where an insulating layer is formed; forming sequentially a gate insulating layer and a body layer on the surface of said insulating layer where said gate electrode is formed; ion implanting into said body layer to form a channel region; forming an impurity-containing layer on said body layer; forming a photoresist pattern on said impurity-containing layer to define a source region, a drain region and a channel region on said body layer; selectively removing said impurity-containing layer using said photoresist pattern as a mask; removing said photoresist pattern; heat treating said impurity-containing layer to form said source, drain and channel regions; and crystallizing said channel region of said body layer by annealing.
 6. A method for manufacturing a thin film transistor as claimed in claim 5, wherein said impurity-containing layer is formed of BPSG in case of a P-type thin film transistor and is formed of PSG in case of an N-type thin film transistor.
 7. A method for manufacturing a thin film transistor as claimed in claim 5, further comprising:washing after said ion implanting to form said channel region.
 8. A method for manufacturing a thin film transistor as claimed in claim 5, further comprising:ion implanting into said body layer to form a lightly doped drain structure after selectively removing said impurity-containing layer.
 9. A method for manufacturing a thin film transistor as claimed in claim 5, wherein said impurity-containing layer is flowed and impurity in the impurity-containing layer is auto-doped into said body layer by said heat treating step to form said source region and said drain region.
 10. A method for manufacturing a thin film transistor as claimed in claim 5, wherein said body layer is formed of polysilicon.
 11. A method for manufacturing a thin film transistor as claimed in claim 5, wherein said impurity-containing layer functions as a micro-lens during crystallizing said channel region of said body layer by said annealing to generate a partial temperature difference at said source region and drain regions and said channel region, and the crystal size at the interface of said source region and said drain region with said channel region is larger than that of said channel region.
 12. A method for manufacturing a thin film transistor as claimed in claim 5, wherein said step of forming the photoresist pattern includes the steps of:depositing a photoresist on said impurity-containing layer, and selectively exposing and developing said photoresist.
 13. A method for manufacturing a think film transistor as claimed in claim 5, wherein said annealing is performed using a prescribed light source.
 14. A method for manufacturing a thin film transistor, comprising the steps of:forming an impurity-containing layer pattern on a body layer to define source, drain and channel regions; flowing said impurity-containing layer pattern covering predetermined portions of said body layer using heat treatment; and crystallizing a portion of said body layer using the flowed impurity-containing layer pattern as a lens member.
 15. A method of manufacturing a thin film transistor as claimed in claim 14, wherein said flowed impurity-containing layer pattern functions as the lens member during crystallizing said body layer so as to generate a partial temperature difference between a first portion of the body layer where said impurity-containing layer pattern is formed and a second portion of the body layer where said impurity-containing layer pattern is not formed.
 16. A method for manufacturing a thin film transistor as claimed in claim 14, wherein the crystal size at an interface of a portion of the body layer where said impurity-containing layer pattern is formed and a portion of the body layer where said impurity-containing layer pattern is not formed, is greater than the crystal size of the body layer where said impurity-containing layer pattern is not formed.
 17. A method for manufacturing a thin film transistor as claimed in claim 14, further comprising:implanting ions into said body layer to form a lightly doped drain structure before said flowing step.
 18. A method for manufacturing a thin film transistor as claimed in claim 14, wherein said impurity-containing layer pattern includes borophospho-silicate glass or phospho-silicate glass.
 19. A method for manufacturing a thin film transistor as claimed in claim 14, wherein said crystallizing is performed by annealing said body layer using a light source or using a rapid thermal processing.
 20. A method for manufacturing a thin film transistor as claimed in claim 14, wherein said body layer is formed of polysilicon. 